In many parts of the world, especially in the near-developed countries such as Taiwan, Korea, Singapore, etc., foamed polyurethanes and glass wools are widely used as sound-shielding and/or fireproofing materials in building constructions and/or interior designs. In these countries, because space is at a premium, the amount of living space that can be allocated to each household is far smaller than that in the U.S, and there are very few of the so-called single family homes. As a consequence, good sound-shielding and fireproof abilities of the construction material are essential for family safety and quiet enjoyment.
Most of the sound-shielding and fireproofing materials such as the foamed polyurethanes and glass wools mentioned above can emit fiber, dust, and other unwelcome particles which can be pathogenic to the human body. Furthermore, these materials are not recyclable, and their wide usage can cause serious word-wide pollution concerns. Therefore, it is paramount that we develop viable substitutes which can minimize or eliminate most, if not all, of these problems, while retaining their sound-shielding and fireproofing capability.
Foamed aluminum materials, or porous aluminum materials, have been developed in recent years as such substitutes. Foamed aluminum materials are formed by adding a foaming agent to a molten aluminum during manufacturing so as to generate gas bubbles therein. The gas bubbles are retained in the molten aluminum during solidification so as to form a highly porous aluminum material. For building construction use, the foamed aluminum typical contains 80% or more of the porous space (i.e., a porosity of at least 80%). Because of their ultra-light weight, and their fireproof and sound-shielding capability, foamed aluminum materials can be used in music halls, disco bars, karaoks, factories, indoor sporting facilities, highway sound shields, automobile bumpers, etc. The manufacturing of a foamed aluminum includes two important considerations. First, the foaming agent must be able to generate the desired amount of gas bubbles of desired sizes. Second, the molten aluminum must possess a certain viscosity so that the gas bubbles generated will be retained in the aluminum matrix during solidification. However, the viscosity of the molten aluminum cannot be too high, so as not to impede the uniform distribution of the gas bubbles.
In Japan Patent Laid-Open Publication JP51-44084, it is disclosed a method by which 1.about.2.5 wt % of a viscosity-enhancing agent magnesium was added to a molten pure aluminum, and 0.15.about.5 wt % of titanium hydride was used as the foaming agent. The foamed aluminum formed according to this method exhibited a porosity of 30.about.60 and a specific density of 0.6.about.1.5. Because of its relatively low porosity, this product is not economically attractive.
In Japan Patent Laid-Open Publication JP54-127838, it is disclosed a method by which the viscosity of the molten aluminum was enhanced by introducing air into the molten aluminum pot, and crystalline water-containing volcanic ash was used as a foaming agent. Since volcanic ash is not widely available, this method has only limited use. Furthermore, large amounts of air are required in order to reach the desired viscosity. This makes the process relatively time consuming and expensive.
In Japan Patent Publication JP57-53425, it is disclosed a method by which the viscosity of the molten aluminum was enhanced by introducing air into the molten aluminum pot, and crystalline water-containing 5CaO.multidot.6SiO.sub.2 .multidot.6H.sub.2 O was used as the foaming agent. The foamed aluminum exhibited a specific density of 0.64, and the pore size (average diameter of the pores) was less than 2 min. Again, large amounts of air are required in order to reach the desired viscosity; this makes the process relatively uneconomical. Furthermore, this process does not achieve high foaming efficiency, and thus is not very commercially attractive.
In Japan Patent Publication JP62-20846, which is contained in the same disclosure as EPO-21803 and U.S. Pat. No. 4,713,277, it is disclosed a method by which 0.2.about.8 wt % calcium was used as the thickener (i.e., viscosity-enhancing agent), and 1.about.3 wt % titanium hydride was used as the foaming agent. The porosity of the foamed aluminum can achieve 85% or better using this method. However, because this method involves the step of adding calcium metal to the molten aluminum pot, it can cause operational difficulties when relatively large size objects (greater than 30.times.30 cm.sup.2) are to be made.
From the above discussions, it is apparent that all the methods that are currently available have their shortcomings, in that they either do not produce foamed aluminums of high enough porosity (i.e., only 30 to 60%, with a specific density between 0.6 and 1.5), or that the methods are not suitable for commercial productions. Furthermore, all these processes disclosed in the prior art require the use of fresh (i.e., new) aluminum material, resulting in relatively high production cost. Therefore, there exists a strong need to develop improved processes for manufacturing foamed aluminum materials that can be used, in an environmentally-conscious and cost-effective manner, to provide sound-shielding and fireproofing in building constructions and interior designs.